1. Field of the Invention
The invention relates to a wire-cut electrical discharge machine having a wire electrode cutting device capable of cutting off a predetermined length of a wire electrode, which is broken by an accident, in order to dispose of the unusable length before rethreading the wire electrode.
2. Description of the Related Art
In the wire-cut electrical discharge machine, electrical discharge pulses are supplied in a machining zone between a wire electrode and a workpiece in order to cut a complex shape from the workpiece. In such a wirecut electrical discharge machine, there is a need to forcibly cut off the wire electrode in two cases when the wire electrode is not broken. For example, in the first case after one workpiece is safely cut and a new workpiece is set, the wire electrode is forcibly cut off. Then, it is easier to position the wire electrode in a starting position for cutting the new workpiece. The second case when the wire electrode needs to be forcibly cut off is when there is one pattern comprising some parts disposed independently each other. To cut out the pattern from the workpiece, first, one part is cut out from the workpiece and then, to cut out a next part from the workpiece, the wire electrode is forcibly cut off. As a result, it is easier to position the wire electrode in a starting position for the next part.
As mentioned above, the electrical discharge pulses are supplied in the machining zone between the wire electrode and the workpiece, so that the pattern is cut out from the workpiece. However, if the electrical discharge repeatedly occurs in the same position of the wire electrode, there is the possibility that the wire electrode will break. Upon the accidental breakage of the wire electrode, a predetermined length of the wire electrode must be cut off before the wire electrode is rethreaded Especially, when the wire electrode breaks by accident, the breaking point of the wire electrode (the pointed head of the wire electrode) or the periphery thereof is usually deformed by the electrical discharges occurring in the machining zone between the wire electrode and the workpiece. As a result, it is difficult to put the pointed head of the wire electrode, resulting from the deformation, into the small wire guide of the wire-cut electrical discharge machine, or into the starting position on the workpiece, that is, it is difficult to rethread the wire electrode in the wire-cut electrical discharge machine.
Therefore, in the wire-cut electrical discharge machine, a predetermined length of the deformed wire electrode is cut off and scrapped. Then, since the wire electrode is smoothly cut at the pointed head, it is easy to introduce the wire electrode into the wire guide so that the wire electrode is rethreaded in the wire-cut electrical discharge machine. Especially, in the case where the wire-cut electrical discharge machine is continuously operated for a long time without an operator, it is preferable that the operation of cutting off and disposing of the unusable length of the broken wire electrode and the operation of rethreading the wire electrode are automatically executed. Various types of wire electrode cutting device capable of automatically executing the above-mentioned operations are disclosed in the art.
One type of wire electrode cutting device is the wire electrode cutting device disclosed in Japanese Laid-Open Patent Publication No. 63-11232. According to the wire electrode cutting device so disclosed, the pointed head of the broken-down wire electrode is held by a clamp (holding means) and the wire electrode is cut off at the wire electrode supply side with respect to the pointed head. After cutting, a driving device lifts and rotates the clamp holding the scrap wire electrode such that the clamp is positioned above a disposal box. When the clamp opens, to release the wire electrode, the scrap wire electrode falls into a disposal box. However, when the clamp opens, the wire electrode does not always release from the clamp. In other words, the operation of disposing of the wire electrode is not always executed with certainty.
Moreover, according to the wire electrode cutting device disclosed in U.S. Pat. No. 4,698,478, the wire electrode to be cut is drawn between a brake device and a clamp device. The brake device is used to give a back tension to the wire electrode, upstream of the feeding direction of the wire electrode, while machining. The rolling-type clamp device is disposed above the workpiece. The drawn wire electrode is cut off by a cutting device disposed above the clamp device and the scrap wire around the clamp is pushed off such as to fall in a disposal chute.
According to the above-described structure, the pointed head of the broken-down wire electrode is wound around the clamp device so as to be drawn tight. Therearound fore, after the wire electrode is cut off, the scrap wire electrode is tightly wound around the clamp device. As a result, it is very difficult to push the bundle of scrap wire from the clump device. Further, a scrap wire push-off disposal device becomes very complex.
In addition, in the above-mentioned wire electrode cutting device, after the unusable length of the wire electrode is cut off, the scrap wire electrode is carried above the disposal box. Therefore, it takes a lot of time to cut off and dispose of the wire electrode. As a result, the wire electrode is not speedily rethreaded and working efficiency is reduced.